Handle reducible nodes

[zspitz]

• [ ] Add option for showing reduced nodes
• [ ] If option is selected, call Reduce recursively and create new models for child nodes
• [ ] Test against ExpressionTreeToString pathspans of Textual tree renderer, to verify that paths are created correctly

Pending https://github.com/zspitz/ExpressionTreeToString/issues/53

[zspitz]

Reduction options:

• Reduce all, i.e. whenever CanREdcue == true
• Reduce Extension nodes
• Don't reduce